The invention relates generally to damping torsional vibrations.
Shaft assemblies in strings of prime movers and loads, comprising turbine generators, electrical motors or compressors, sometimes exhibit weakly damped mechanical resonances (torsional modes) with frequencies which can also fall below the network synchronous frequency. Mechanical damage can occur over time if an electric network exchanges significant energy with a generator at one or more of the natural frequencies of the shaft systems. Conventional countermeasures for torsional resonance phenomena include efforts to eliminate the source of resonance excitation by, for example, changing the network, operational, or control parameters.
When a power system has a connection to a large utility network with many gigawatts of generated power, electrical loads on the power system have only a negligible effect. In contrast, island power systems have no connection to a large utility network and island-like power systems have only a weak connection to a utility network (such as by means of a long transmission line with a comparably high impedance). Island and island-like power systems are commonly used in the marine industry (for example, onboard power systems of large ships), isolated on-shore installations (for example, wind turbine systems), and the oil and gas industry. In such power systems, the loads are typically large relative to the network (such that the loads can potentially affect the network). With increasing ratings of electrical motors and drives in these systems, the mechanical and electrical dynamics are increasingly coupled, thus making it difficult to avoid torsional oscillations in motor or generator drive trains by conventional countermeasure techniques.
One method for suppressing torsional vibrations in synchronous generators is described in C. Sihler, “Suppression of torsional vibrations in rotor shaft systems by a thyristor controlled device,” 35th Annual IEEE Power Electronics Specialist Conference, pages 1424–1430 (2004). The method includes applying a torque in counter phase to a measured torsional velocity by means of an additional thyristor converter circuit comprising a six-pulse bridge circuit. This method is most applicable to embodiments wherein the installation of a separate line commutated converter system is technically and economically feasible.